Simplifying Trigonometric Identities

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Question 1 of 7

Which of the following are trigonometric identities?

1. ${csc}^{2} θ = {cot}^{2} θ + 1$ $\text(csc)^2 theta=\text(cot)^2 theta +1$
2. ${cot}^{2} θ = {cos}^{2} θ + 1$ $\text(cot)^2 theta=\text(cos)^2 theta+1$
3. ${sec}^{2} θ = 1 + {sin}^{2} θ$ $\text(sec)^2 theta=1+\text(sin)^2 theta$
4. ${sec}^{2} θ = 1 + {tan}^{2} θ$ $\text(sec)^2 theta=1+\text(tan)^2 theta$

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Trigonometric Identities

sec θ = \frac{1}{cos θ}

$\sec\theta=\frac{1}{\text{cos}\theta}$

csc θ = \frac{1}{sin θ}

$\csc\theta=\frac{1}{\text{sin}\theta}$

cot θ = \frac{1}{tan θ}

$\cot\theta=\frac{1}{\text{tan}\theta}$

tan θ = \frac{sin θ}{cos θ}

$\tan\theta=\frac{\text{sin}\theta}{\text{cos}\theta}$

cot θ = \frac{cos θ}{sin θ}

$\cot\theta=\frac{\text{cos}\theta}{\text{sin}\theta}$

Using the given image, we can see that the radius of the circle will be

1

$1$ . Therefore, the circle follows the formula

x^{2} + y^{2} = 1

$x^2+y^2=1$ , where

1

$1$ is the radius.

Make a reference triangle by making a line from the point of origin

(0, 0)

$(0,0)$ going to any point in the circle to represent the radius and connect it to either the x or y-axis

Notice that, given angle

θ

$theta$ , the circle has the vertical line parallel to the y-axis as its opposite side and the line on the x-axis as the adjacent side

Now that we have the opposite and adjacent sides and the radius

1

$1$ as the hypotenuse, we can use the trigonometric functions to find their respective values

$sin$ $\text(sin)$	$=$ $=$	$\frac{opposite}{hypotenuse}$ $\text(opposite)/(\text(hypotenuse))$

	$=$ $=$	$\frac{y}{1}$ $y/1$

	$=$ $=$	$y$ $y$

$cos$ $\text(cos)$	$=$ $=$	$\frac{adjacent}{hypotenuse}$ $\text(adjacent)/(\text(hypotenuse))$

	$=$ $=$	$\frac{x}{1}$ $x/1$

	$=$ $=$	$x$ $x$

x

$x$ and

y

$y$ are both coordinate values, so we can write the coordinates as

(cos θ, sin θ)

$(\text(cos) theta,\text(sin) theta)$

Finally, use the values of

x

$x$ and

y

$y$ and derive its trigonometric identities

$x^{2} + y^{2}$ $x^2+y^2$	$=$ $=$	$1$ $1$
${cos}^{2} θ + {sin}^{2} θ$ $\text(cos)^2theta+\text(sin)^2theta$	$=$ $=$	$1$ $1$	Substitute known values
$({cos}^{2} θ + {sin}^{2} θ)$ $(\text(cos)^2theta+\text(sin)^2theta)$ $\div {cos}^{2} θ$ $divide \text(cos)^2theta$	$=$ $=$	$1$ $1$ $\div {cos}^{2} θ$ $divide \text(cos)^2theta$	Divide both sides by ${cos}^{2} θ$ $\text(cos)^2theta$

$1 + \frac{{sin}^{2} θ}{{cos}^{2} θ}$ $1+(\text(sin)^2theta)/(\text(cos)^2theta)$	$=$ $=$	$\frac{1}{{cos}^{2} θ}$ $1/(\text(cos)^2theta)$

$1 +$ $1+$ ${tan}^{2} θ$ $\text(tan)^2theta$	$=$ $=$	$\frac{1}{{cos}^{2} θ}$ $1/(\text(cos)^2theta)$	$\frac{{sin}^{2} θ}{{cos}^{2} θ} = {tan}^{2} θ$ $(\text(sin)^2theta)/(\text(cos)^2theta)=\text(tan)^2theta$

$1 + {tan}^{2} θ$ $1+\text(tan)^2theta$	$=$ $=$	${sec}^{2} θ$ $\text(sec)^2theta$	$\frac{1}{{cos}^{2} θ} = {sec}^{2} θ$ $1/(\text(cos)^2theta)=\text(sec)^2theta$
${sec}^{2} θ$ $\text(sec)^2theta$	$=$ $=$	$1 + {tan}^{2} θ$ $1+\text(tan)^2theta$

$x^{2} + y^{2}$ $x^2+y^2$	$=$ $=$	$1$ $1$
${cos}^{2} θ + {sin}^{2} θ$ $\text(cos)^2theta+\text(sin)^2theta$	$=$ $=$	$1$ $1$	Substitute known values
$({cos}^{2} θ + {sin}^{2} θ)$ $(\text(cos)^2theta+\text(sin)^2theta)$ $\div {sin}^{2} θ$ $divide \text(sin)^2theta$	$=$ $=$	$1$ $1$ $\div {sin}^{2} θ$ $divide \text(sin)^2theta$	Divide both sides by ${sin}^{2} θ$ $\text(sin)^2theta$

$\frac{{cos}^{2} θ}{{sin}^{2} θ} + 1$ $(\text(cos)^2theta)/(\text(sin)^2theta)+1$	$=$ $=$	$\frac{1}{{sin}^{2} θ}$ $1/(\text(sin)^2theta)$

${cot}^{2} θ$ $\text(cot)^2theta$ $+ 1$ $+1$	$=$ $=$	$\frac{1}{{sin}^{2} θ}$ $1/(\text(sin)^2theta)$	$\frac{{cos}^{2} θ}{{sin}^{2} θ} = {cot}^{2} θ$ $(\text(cos)^2theta)/(\text(sin)^2theta)=\text(cot)^2theta$

${cot}^{2} θ + 1$ $\text(cot)^2theta+1$	$=$ $=$	${csc}^{2} θ$ $\text(csc)^2theta$	$\frac{1}{{sin}^{2} θ} = {csc}^{2} θ$ $1/(\text(sin)^2theta)=\text(csc)^2theta$
$\text(csc)^2theta$	$=$	$\text(cot)^2theta+1$

$\text(sec)^2theta=1+\text(tan)^2theta$

$\text(csc)^2theta=\text(cot)^2theta+1$

Question 2 of 7

Simplify

$1/(1+sin theta)+1/(1-sin theta)$

1. $\text(csc)^2theta+2$
2. $\text(csc)^2theta$
3. $2/(\text(sec)^2theta)$
4. $2\text(sec)^2theta$

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Trigonometric Identities

$\sec\theta=\frac{1}{\text{cos}\theta}$

$\csc\theta=\frac{1}{\text{sin}\theta}$

$\cot\theta=\frac{1}{\text{tan}\theta}$

$\tan\theta=\frac{\text{sin}\theta}{\text{cos}\theta}$

$\cot\theta=\frac{\text{cos}\theta}{\text{sin}\theta}$

Apply the rule of adding fractions and simplify the expression

	$1/(1+\text(sin) theta)+1/(1-\text(sin) theta)$

$=$	$(1-\text(sin) theta+1+\text(sin) theta)/((1+\text(sin) theta)(1-\text(sin) theta))$

$=$	$2/((1+\text(sin) theta)(1-\text(sin) theta))$	Combine like terms

$=$	$2/(1-\text(sin)^2theta)$	Expand

Recall that

$\text(sin)^2theta+\text(cos)^2theta=1$ . Derive this formula to further simplify the expression

$\text(sin)^2theta+\text(cos)^2theta$	$=$	$1$
$\text(sin)^2theta+\text(cos)^2theta$ $-\text(sin)^2theta$	$=$	$1$ $-\text(sin)^2theta$	Subtract $\text(sin)^2theta$ from both sides
$\text(cos)^2theta$	$=$	$1-\text(sin)^2theta$

	$2/(1-\text(sin)^2theta)$

$=$	$2/(\text(cos)^2theta)$	$1-\text(sin)^2theta=\text(cos)^2theta$

$=$	$2/1xx 1/(\text(cos)^2theta)$	Take out $2$

$=$	$2xx\text(sec)^2theta$	$1/(\text(cos)^2theta)=\text(sec)^2theta$
$=$	$2\text(sec)^2theta$

$2\text(sec)^2theta$

Question 3 of 7

Simplify

$(tan theta)/(sec theta)$

1. $1/(\text(tan) theta)$
2. $\text(sin) theta$
3. $2\text(sec) theta$
4. $\text(cos) theta$

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Trigonometric Identities

$\sec\theta=\frac{1}{\text{cos}\theta}$

$\csc\theta=\frac{1}{\text{sin}\theta}$

$\cot\theta=\frac{1}{\text{tan}\theta}$

$\tan\theta=\frac{\text{sin}\theta}{\text{cos}\theta}$

$\cot\theta=\frac{\text{cos}\theta}{\text{sin}\theta}$

Substitute known values to

$\text(tan)$ and

$\text(sec)$

$\text(tan)$	$=$	$(\text(sin))/(\text(cos))$

$\text(sec)$	$=$	$1/(\text(cos))$

	$(\text(tan) theta)/(\text(sec) theta)$

$=$	$((\text(sin))/(\text(cos)))/(1/(\text(cos)))$	Substitute known values

$=$	$\text(sin) theta$	Apply rule of dividing fractions

$\text(sin) theta$

Question 4 of 7

Simplify

$sec theta-sec theta sin^2theta$

1. $\text(csc) theta$
2. $\text(sin) theta$
3. $\text(sec) theta$
4. $\text(cos) theta$

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Trigonometric Identities

$\sec\theta=\frac{1}{\text{cos}\theta}$

$\csc\theta=\frac{1}{\text{sin}\theta}$

$\cot\theta=\frac{1}{\text{tan}\theta}$

$\tan\theta=\frac{\text{sin}\theta}{\text{cos}\theta}$

$\cot\theta=\frac{\text{cos}\theta}{\text{sin}\theta}$

Factor out

$\text(sec) theta$ on both terms

		$\text(sec) theta-\text(sec) theta \text(sin)^2theta$
	$=$	$\text(sec) theta(1-\text(sin)^2theta)$	Factorise

Recall that

$\text(sin)^2theta+\text(cos)^2theta=1$ . Derive this formula to further simplify the expression

$\text(sin)^2theta+\text(cos)^2theta$	$=$	$1$
$\text(sin)^2theta+\text(cos)^2theta$ $-\text(sin)^2theta$	$=$	$1$ $-\text(sin)^2theta$	Subtract $\text(sin)^2theta$ from both sides
$\text(cos)^2theta$	$=$	$1-\text(sin)^2theta$

	$\text(sec) theta(1-\text(sin)^2theta)$
$=$	$\text(sec) theta(\text(cos)^2theta)$	$1-\text(sin)^2theta=\text(cos)^2theta$

$=$	$1/(\text(cos) theta)*(\text(cos)^2theta)$	$\text(sec) theta=1/(\text(cos) theta)$

$=$	$(\text(cos)^2theta)/(\text(cos) theta)$

$=$	$\text(cos) theta$

$\text(cos) theta$

Question 5 of 7

5. Question
Simplify

$sec^2A-csc^2A$
- 1. $(\text(sec) A-\text(csc) A)(\text(sec) A-\text(csc) A)$
- 2. $(\text(sec) A-\text(csc) A)(\text(sec) A+\text(csc) A)$
- 3. $(\text(sec) A+\text(csc) A)(\text(sec) A+\text(csc) A)$
- 4. $A(\text(sec) A+\text(csc) A)$
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The difference of two squares is a format of a quadratic expression where there are two terms that are both perfect squares.

First, find the square of the two terms.

First term

$\text(sec)^2A$ $=$ $\text(sec) A*\text(sec) A$

Second term

$\text(csc)^2A$ $=$ $\text(csc) A*\text(csc) A$

Finally, get the sum and difference of the two values.

$($ $\text(sec) A$ $-$ $\text(csc) A$ $)($ $\text(sec) A$ $+$ $\text(csc) A$ $)$

$(\text(sec) A-\text(csc) A)(\text(sec) A+\text(csc) A)$

Question 6 of 7

Simplify

$(tan^2theta)/(sec theta-1)$

1. $\text(sec)theta+1$
2. $\text(sec)theta-1$
3. $-1/(\text(sec)theta)$
4. $1/(\text(sec)theta)$

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Trigonometric Identities

$\sec\theta=\frac{1}{\text{cos}\theta}$

$\csc\theta=\frac{1}{\text{sin}\theta}$

$\cot\theta=\frac{1}{\text{tan}\theta}$

$\tan\theta=\frac{\text{sin}\theta}{\text{cos}\theta}$

$\cot\theta=\frac{\text{cos}\theta}{\text{sin}\theta}$

Recall that

$1+\text(tan)^2theta=\text(sec)^2theta$ . Derive this formula to further simplify the expression

$1+\text(tan)^2theta$	$=$	$\text(sec)^2theta$
$1+\text(tan)^2theta$ $-1$	$=$	$\text(sec)^2theta$ $-1$	Subtract $1$ from both sides
$\text(tan)^2theta$	$=$	$\text(sec)^2theta-1$

	$(\text(tan)^2theta)/(\text(sec) theta-1)$

$=$	$(\text(sec)^2theta-1)/(\text(sec) theta-1)$	$\text(tan)^2theta=\text(sec)^2theta-1$

$=$	$((\text(sec)theta-1)(\text(sec)theta+1))/(\text(sec) theta-1)$	Factorise

$=$	$\text(sec)theta+1$	$(\text(sec) theta-1)/(\text(sec) theta-1)=1$

$\text(sec)theta+1$

Question 7 of 7

Simplify

$((tan^2beta)(cot^2beta+1))/(tan^2beta+1)$

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Trigonometric Identities

$\sec\theta=\frac{1}{\text{cos}\theta}$

$\csc\theta=\frac{1}{\text{sin}\theta}$

$\cot\theta=\frac{1}{\text{tan}\theta}$

$\tan\theta=\frac{\text{sin}\theta}{\text{cos}\theta}$

$\cot\theta=\frac{\text{cos}\theta}{\text{sin}\theta}$

Recall that

$1+\text(cot)^2theta=\text(csc)^2theta$ . Derive this formula to further simplify the expression

	$((\text(tan)^2beta)(\text(cot)^2beta+1))/(\text(tan)^2beta+1)$

$=$	$(\text(tan)^2beta*\text(csc)^2theta)/(\text(tan)^2beta+1)$	$1+\text(cot)^2theta=\text(csc)^2theta$

Finally, substitute known values to

$\text(tan)$ ,

$\text(csc)$ , and

$\text(sec)$

$\text(tan)$	$=$	$(\text(sin))/(\text(cos))$

$\text(csc)$	$=$	$1/(\text(sin))$

$\text(sec)$	$=$	$1/(\text(cos))$

	$(\text(tan)^2beta*\text(csc)^2theta)/(\text(tan)^2beta+1)$

$=$	$((\text(sin)^2beta)/(\text(cos)^2beta)*1/(\text(csc)^2theta))/(1/(\text(cos)^2theta))$	Substitute known values

$=$	$(\text(sin)^2beta)/(\text(cos)^2beta)1/(\text(csc)^2theta)(\text(cos)^2theta)/1$	Apply rule of dividing fractions

$=$	$1$

$1$

Simplifying Trigonometric Identities

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